This invention relates to a pneumatic radial tire for passenger cars designed to improve steering stability while achieving weight reduction of the tread-belt structure.
The recent global spread of environmental pollution has produced a strong demand for more and more fuel-efficient vehicles. One approach, weight reduction of tires, is attracting attention as a major technological challenge.
Among pneumatic redial tires for passenger cars, those in which the belt plies are built of steel cords are known to produce high steering stability. This is because steel cords possess very high strength and resilience compared with other cords of fabrics. However, the steel cords, with greater specific weight, add more to the weight of tires and hence to the fuel cost. They therefore can hardly solve the above-mentioned technical problem.
As a new tire cord material almost comparable to steel cords in characteristics, aramid fiber cords have been proposed. Aramid fiber cords have practically as high strength and resilience as steel cords, but are lighter in specific weight and can contribute to the weight reduction of tires. It has been found, for example, that mere replacement of belt plies of steel cords by those of aramid fiber cords lessens the tire weight by about 5 to 8 percent.
Aramid fiber cords, whose compression rigidity is almost zero, have the disadvantage of exhibiting low flexural rigidity upon exposure to bending deformation. Cornering power given by a tire with belt plies using aramid fiber cords in place of steel cords of the same structure is at most 75 percent of the power obtained with a tire having steel-cord belt plies. With aramid fiber cords, therefore, it has been believed practically impossible to enhance the steering stability to the level attainable with a tire of the belt structure using steel cords.